The sampling oscilloscope is among the measurement devices used to determine the modulation waveform of modulated optical signals such as those used in optical telecommunications. Conventional sampling oscilloscopes incorporate a conventional optical signal sampling device that first converts the optical signal into an electrical signal using a photodetector. The waveform of the electrical signal is then measured using an electrical sampling circuit.
FIG. 1 is a block diagram of an example of a conventional optical signal sampling device 10 for sampling an optical signal. The optical signal sampling device is composed of the photodetector 12, the electrical sampling circuit 14 and the optical signal input 16. The optical signal-under-test is received via the optical signal input. In this example, the optical signal input is composed of the input fiber 18 and the converging element 20. The converging element converges the optical signal-under-test received via the input fiber onto the photodetector. The photodetector generates an electrical signal in response to the optical signal-under-test, and the electrical signal is sampled by the electrical sampling circuit.
The maximum frequency capability of the photodetector 12 needs to be several times the maximum frequency of the modulation waveform of the optical signal-under-test. In the example shown, the photodetector includes the photodiode 22. The photodiode is DC biased by the DC bias source 24. Currently-available PIN diodes suitable for use as the photodiode have a maximum frequency of about 110 GHz. The photodetector may alternatively include an avalanche photodiode, but an avalanche photodiode has a maximum frequency similar to that of a PIN diode.
The electrical sampling circuit 14 is composed of the sampling element 30, the sampling pulse generator 32 and the clock pulse generator 34. The sampling element includes the electrical signal input 42, the sampling signal input 44 and the electrical sample output 46. The sampling element is typically composed of a resistor, a capacitor, and a diode, none of which is shown. The output of the clock pulse generator is connected to the input of the sampling pulse generator. The output of the sampling pulse generator is connected to the sampling signal input 44 of the sampling element. The electrical signal input 42 of the sampling element is connected to the output of the photodetector 12. The electrical sample output 46 of the sampling element provides electrical samples each of which represents a portion of the modulation waveform of the optical signal-under-test. Also shown are the integrating capacitor 50 and the buffer amplifier 52 that respectively integrate and buffer the electrical samples generated by the electrical sampling circuit 14 to generate an electrical output signal whose waveform represents the modulation waveform of the optical signal-under-test.
The clock pulse generator 34 generates electrical pulses having a duration of a few nanoseconds. The sampling pulse generator 32 compresses the electrical pulses received from the clock pulse generator to generate electrical sampling pulses having a duration of a few tens of picoseconds. The sampling pulse generator may include, for example, non-linear transmission line (NLTL), a step recovery diode and a strip line, none of which is shown. The sampling pulse generator supplies the sampling pulses to the sampling element 30.
Currently, the maximum modulation frequency that can be measured using a sampling oscilloscope incorporating the conventional optical signal sampling device as just described is about 50 GHz. As the capacity of optical telecommunications has grown in recent years, the modulation rates of optical signals have reached 80 Gbps or more. It is predicted that sampling devices for measuring the waveforms of optical signals modulated at such high modulation rates will need a frequency response that extends to over 200 GHz. It is difficult to sample optical signals with such high modulation rates using conventional electrical sampling. A frequency response that extends to over 200 GHz needs samples having a duration of about 2 ps. It is extremely difficult to generate the electrical sampling pulses required to generate such short-duration electrical samples using a conventional NLTL. Moreover, propagation losses in the strip line become significant at such high frequencies. Therefore, a need exists for an optical signal sampling device capable of accurately sampling a modulated optical signal, and especially an optical signal modulated at a modulation frequency in the frequency range in which electrical sampling is difficult or impossible.
In addition, many optical telecommunication systems employ wavelength-division multiplexing (WDM). WDM increases the transmission capacity of an optical telecommunication system without increasing the transmission rate. In WDM, a transmission channel carries many optical signals, each of a different wavelength. What is also needed is the ability to measure the modulation waveforms of the individual optical signals, each of which has a different wavelength and a different modulation waveform.
The invention provides an optical domain optical signal sampling device for sampling an optical signal-under-test. The optical domain optical signal sampling device comprises a semiconductor saturable absorber, an optical sampling pulse source, an optical signal input and a light detector. The optical sampling pulse source is arranged to illuminate the semiconductor saturable absorber with optical sampling pulses. The optical signal input is arranged to illuminate the semiconductor saturable absorber with the optical signal-under-test. The light detector is arranged to receive optical samples of the optical signal-under-test output by the semiconductor saturable absorber in response to the optical sampling pulses.
The semiconductor saturable absorber enables the optical domain optical signal sampling device to sample the optical signal-under-test in the optical domain. Optical sampling pulse sources capable of repetitively generating pulses of light having a duration of a few picoseconds and with an intensity sufficiently high to induce saturable absorption in the semiconductor saturable absorber are commercially available. When illuminated with such short optical sampling pulses and the optical signal-under-test, the semiconductor saturable absorber outputs optical samples of the optical signal-under-test. The optical samples have a duration similar to that of the optical sampling pulses. The light detector converts the optical samples into an electrical signal that represents the modulation waveform of the optical signal-under-test. The frequency response of the light detector need not extend significantly higher in frequency than that of the modulation waveform of the optical signal-under-test. Such light detectors are also commercially available. Thus, the optical domain optical signal sampling device according to the invention can sample modulated optical signals and can provide a frequency response extending to beyond 200 GHz.
The optical sampling pulse source may include a laser that generates pulses of light having a pulse width of no more than 10 ps and the semiconductor saturable absorber may include a semiconductor material with a carrier lifetime of no more than 10 ps.
The semiconductor saturable absorber may include a semiconductor layer comprising semiconductor material having a short carrier lifetime. Semiconductor material having a short carrier lifetime generates the optical samples with a duration comparable with that of the optical sampling pulses and increases the wavelength range over which saturable absorption is induced. The semiconductor material having a short carrier lifetime may include low-temperature-deposited semiconductor material or ion implanted semiconductor material, for example.
The semiconductor layer may be a layer of bulk semiconductor material or a layer of graded-composition semiconductor material. Alternatively, the semiconductor layer may have a multiple quantum well structure.
The light detector may be arranged to receive the optical samples of the optical signal-under-test transmitted through the semiconductor saturable absorber. Alternatively, the semiconductor saturable absorber may include a reflective layer and the light detector is arranged to receive the optical samples of the optical signal-under-test reflected through the semiconductor saturable absorber by the reflective layer.
The light detector generates an electrical signal in response to the optical samples of the optical signal-under-test, and the optical domain optical signal sampling device may additionally comprise elements that increase the signal-to-noise ratio of the electrical signal.
In embodiments in which the optical signal-under-test is linearly polarized and has a direction of polarization, the elements for increasing the signal-to-noise ratio may include a polarization analyzer and an element that linearly polarizes the optical sampling pulses with a direction of polarization orthogonal to the direction of polarization of the optical signal-under-test. The polarization analyzer is interposed between the semiconductor saturable absorber and the light detector, and has a polarization direction of maximum transmission parallel to the direction of polarization of the optical signal-under-test. Alternatively, the elements for increasing the signal-to-noise ratio may include quantum wells in the semiconductor saturable absorber, a polarization analyzer and an element that circularly polarizes the optical sampling pulses. The polarization analyzer is interposed between the semiconductor saturable absorber and the light detector, and has a polarization direction of maximum transmission orthogonal to the direction of polarization of the optical signal-under-test.
In embodiments in which the optical signal-under-test has an indeterminate polarization, the elements for increasing the signal-to-noise ratio includes quantum wells in the semiconductor saturable absorber; elements that derive left-handed and right-handed circularly polarized optical sampling pulse components from the optical sampling pulses and illuminate a first region and a second region of the semiconductor saturable absorber with the left-handed and the right-handed circularly polarized optical sampling pulse components, respectively; elements that derive from the optical signal-under-test a first polarization component and a second polarization component having orthogonal directions of polarization, and that illuminate the first region and the second region of the semiconductor saturable absorber with the first and the second polarization components, respectively; and first and second polarization analyzers that are interposed between the semiconductor saturable absorber and the light detector in the path of optical samples generated from the first and the second polarization components, respectively, and that each have a respective polarization direction of maximum transmission orthogonal to the direction of polarization of the first and the second polarization components, respectively.